The increasing demand for capacity in cellular systems has led to the 3GPP (3rd Generation Partnership Project) mobile system standard evolving into what is called LTE (Long Term Evolution) and LTE Advanced. This has opened up for the use of more advanced antenna technology. In particular, antenna array techniques such as MIMO (Multiple Input Multiple Output) or beam-forming are often applied.
Antenna arrays have previously been used in radar and satellite communication applications. In order to form beams, an antenna array has to be phase aligned or calibrated at the antenna ports. This is often a time consuming and expensive measurement process performed once at an antenna measurement range before deployment. However, commercial antennas in the mobile radio industry drive requirements for a self-contained swift calibration method that also can be performed regularly on-line, i.e. during operation.
Several proposals for calibration methods exist in the industry today. It is known in the art how to inject a calibration signal into the TX radio chains and sample a small portion of the radio signals close to the antenna ports, e.g. using a directional coupler. By feeding the sampled output signal and the TX radio chain input signal (including the injected calibration signal) to a signal correlator, a phase, gain and delay correction could be calculated and inserted in the radio chain, preferably in the digital base-band part. A general method for this type of antenna calibration is described in U.S. Pat. No. 6,339,399. However, methods of this type incorporate a network of calibration couplers connected by switches or summing resistors to some calibration receiver. These calibration chains will introduce errors if not properly phase aligned by design or properly characterized and corrected for. Methods of this type simply presuppose the calibration network characteristics to be known, which is a significant requirement.
Another method known in the art is applicable to TDD (Time Division Multiplexing) systems only. The idea is to use the antenna array mutual coupling in order to get a calibration signal back into the system for correlation and calculating the required corrections for both RX and TX radio chains. RX and TX radio chains are used alternately in TDD, but this type of calibration requires some of the TX and RX chains to be used simultaneously and not alternately, forcing the insertion of dedicated calibration time periods devoid of normal radio traffic. This type of calibration also presupposes uniformity in the mutual coupling between antenna elements. A method along these lines is described in U.S. Pat. No. 5,657,023. However, the described method is restricted to TDD applications, leaving the large area of FDD (Frequency Division Multiplexing) unsolved. Furthermore, the described method presupposes uniformity in the antenna mutual coupling. Another drawback is the need to insert dedicated calibration time periods into the regular radio traffic.